The present application relates to an integrated day and night sighting system and to a laser rangefinder having the integrated day and night sighting system. This invention addresses the need for minimum size and weight in handheld laser rangefinders used by military ground forces in nighttime as well as daytime conditions. Integrated optics are needed to provide the following functions: a telescopic day vision sight; telescopic night vision sight; sighting reference (e.g., crosshair reticle) for both day and night operations; laser beam divergence reduction; and target return signal collection and focusing onto a laser ranging receiver.
Telescopic day vision is usually achieved with direct view optics such as used in conventional telescopes and binoculars. These optics operate in the human visual light spectrum of 0.4 to 0.7 um wavelength. In their simplest form they comprise an objective lens that focuses the scene image onto the focal plane of an eyepiece. The ratio of objective focal length to eyepiece focal length establishes the magnification. To present a non-inverted image to the user, an imaging erection prism is added between the objective lens and the focal plane.
Telescopic night vision is achieved with an image converter, which can be an image intensifier or a thermal imaging module. Objective optics collect the scene light and focus it onto the image converter. The input aperture of these optics needs to be as large as possible (small f-number) to maximize image intensity at the image converter. The image intensifier is a single component in which a weak (moonlit, starlit) scene image focused on the input photocathode produces electrons that are amplified and directed to a phosphor coating on the output surface where they produce an amplified image for viewing with an eyepiece. This operates in the 0.7-1.1 um wavelength region. The simplest thermal imaging module comprises an uncooled, staring (non-scanned) detector array whose output is presented to a flat panel display that is viewed with an eyepiece. This operates in the 8 to 14 um wavelength region. Other implementations of a thermal imaging module can operate in the 3 to 5 um wavelength region, employ thermoelectric or Stirling cooled detectors, and/or use a cathode ray tube (CRT) as the display.
To aim a laser rangefinder requires a sighting reference, such as a crosshairs reticle, that boresights the day and night vision optics to the laser beam. The reticle can take a number of forms. A day sight can use a passive reticle plate residing in the common focal planes of the objective and eyepiece. An image intensified sight can produce the reticle with a light emitting diode (LED) array and project it into the eyepiece using a beamsplitter to combine the image intensifier image and the reticle. A thermal imaging sight can produce the reticle on the flat panel or CRT display. The two latter forms of the reticle also allow alphanumeric data and/or indicators to be displayed without additional elements.
The output beam divergence of a laser transmitter (resonator) is not usually narrow enough to produce a small spot on a distant target such as a vehicle so that an afocal beam collimation telescope is necessary. This comprises a negative power input lens and a positive power output lens. The resulting output beam diameter is enlarged and the beam divergence is reduced. To avoid possible eye damage to a person accidentally exposed to the beam, the laser wavelength is commonly in the 1.5 um region, for which the eye has poor transmission.
The laser receiver requires optics to collect return signal power from the target and focus it onto the receiver's detector. Maximum input aperture results in maximum range capability. The receiver incorporates an optical filter to minimize sunlight detection and thereby maximize receiver sensitivity.
The above functions can be achieved with separate apertures and corresponding optical arrangements. However, striving for a single aperture and using optical elements for more than one function minimizes the size of the optics. That is the aim of this invention. The invention is applicable not only to a handheld laser rangefinder but to any rangefinder and to any day and night sighting system.
The Phillips U.S. Pat. No. 5,084,780, for a telescopic sight for day/night viewing illustrates a scope adaptable for use on weapons and includes a single objective lens with two parallel light paths, one for day viewing and one for night viewing. Separating mirrors transmit light from the objective along the night path and reflects light from the objective lens to the day path. The night path includes an image intensifier assembly. A mirror at the end of the night path reflects the light from the image intensifier assembly to a beam splitter on the day path. The beam splitter transmits the light from the day path and reflects the light from the night path along the same path to an ocular assembly for viewing. A second embodiment is similar except that it contains two objective lens assemblies for collecting the light, one for the night path and one for the day path. Since this embodiment has two separate objective lens assemblies, separating mirrors are not included. A third embodiment includes a projected aiming reticle in a direct view for day viewing, which replaces one of the objective lens assemblies. This prior patent combines the daytime and nighttime optics in a telescopic sight by separating the optics into two separate paths between the objective lens and the eyepiece assembly. In contrast, the present invention uses a coaxially mounted lens with a night light mirror objective lens in the same optic path for the day and night light.